Electric heating system



2 Sheets-Sheet l |NvENToR James L. Reed.

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42) ee 2o J. L. REED ELECTRIC HEATING SYSTEM IIO Sept. 18, 1951` Filed March 9, 1949 IOO los

ATTORNEY Sept. 18, 1951 J, L REED 2,568,411

ELECTRIC HEATING SYSTEM |74 |84 [64 o c l|62 |60/ Fig. 5 D

l:Il:l ,20e i200 muur-|92 |86 |68/ Il |96 C /204 mmm w|TNEssEs lNvENToR James L. Reed. @M BY A @0 am@ @2247222 ATTORNEY Patented Sept. 18, 1951 ELECTRIC HEATING SYSTEM James L. Reed, Buffalo, N. Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania- Application March 9, 1949, Serial No. 80,404

(Cl. 21S-20) 18 Claims.

My invention relates to electric heating means having electric heating resistors that heat the heating means when electric current is passed through them. More particularly, my invention relates to resistor-type electric furnaces provided with temperature-responsive means and a regulator controlled thereby for regulating the supply of electric power to the heating resistors so as to maintain the temperature of the furnace at some desired level or predetermined value with a high degree of accuracy.

In more conventional types of furnace systems to which my invention pertains, the power supplied to the furnace heating resistors is controlled by controlling the voltage across the heating resistors or by yconnecting them in different series, parallel, and series-parallel circuit arrangements. In the former instance, a voltage-controlling device is connected to the heating resistors for controlling the voltage across them. For suitable and accurate temperature control of the furnace, such a device must be able to regulate the power delivered to the heating resistors when the demand for heat is exceptionally high, and also when the demand for heat is fairly constant but relatively low. The former situation usually ocurs when a cold furnace is being brought into operation; and the latter usually occurs when a heated charge is being soaked in the furnace or is being permitted to change slowly in temperature. Accordingly, the regulating devices of conventional electric furnace systems must have an operating range that extends from the high power supplied during, for example, the furnace heatingup periods, to the much lower power supplied during, for example, soaking or operating periods. Such a regulating means can be fairly expensive in cost.

An object of my invention is to provide an electric furnace system or equivalent electric heating means of a type described in which different -circuit arrangements for the heating resistors are combined with a regulating means in such a manner that the regulating means is not called upon to carry the high currents or high power that may sometimes be delivered to the heating resistors, so that a smaller regulating means can be used than would be the case otherwise. I accomplish this object by limiting the current and voltage values which the regulating means is required, at any time, to handle. The magnitudes are much lower than the full current-rating or full voltage-rating of the heating resistors of the electrical furnace to which the invention is applied. Consequently, a fiexible heating system embodying my invention can be installed at less cost than a similar heating system of the same rating using a regulating means having a K. V. A. rating for handling the maximum rated capacity of the heating resistors.

For illustrating my invention, I show a regulating system of a type using a saturable core reactor having a variable impedance winding that can be connected in series with the heating resistors of the furnace. The impedance of this winding determines the voltage across the heating resistors. A system of this kind is shown in the patent to K. P. Puchlowski, No. 2,445,454, dated July 20, 1948 In said system, line voltage is applied across a series circuit comprising the A. C. winding of the saturable core reactor and the heating resistors of an electric furnace. By varying the current in the D. C. control winding of the saturable core reactor in accordance with the temperature of the furnace, the A. C. winding can be made to oier more impedance when the heat demanded by the furnace is relatively less, and vice versa. Consequently, the voltage across and the current through the heating resistors are regulated in accordance with the temperature. of the furnace. Because the A. C. winding is permanently in series with the heating resistors, it must be designed to carry the full-rated current of the heating resistors when the impedance of the A. C. winding is at a minimum, and also to withstand almost the full-rated voltage when the impedance is at a maximum.

In accordance with my invention, the heating resistors of a furnace are divided into several sections of heating resistors. The sections can be connected selectively with all of them in a series circuit arrangement across the power supply line. The A. C. winding of a saturable core reactor is used for regulating purposes only when the full desired number of heating resistors are in the series-circuit arrangement. In the seriescircuit arrangement, the maximum current that can be carried by the heating resistors is limited to a value much less than the current that can be carried by the resistors in a parallel-circuit or other arrangement; and a saturable core reactor of much lower rating can be used with corresponding savings through the system. In specific forms of my invention, for high heat demand the heating resistors are connected directly, in Y or in delta, to'all phases of a three phase line, without the regulating reactor. When the heat demand becomes less, the resistors are 3 connected in series with the reactor across a single phase of the line.

Further objects, features and innovations of my invention will be apparent from the following description of several preferred embodiments thereof. The principles of my invention are, however, not limited thereto. The description is to be taken in conjunction with the accompanying schematic drawings, in which:

Figure 1 is a wiring diagram of an embodiment illustrating a form of circuit connections by means of which the heating resistors of an Velectric furnace heating means can be controlled in accordance with my invention;

Figi. 2 is a wiring diagram that complements Fig. 1, and shows a scheme by which circuits shown in Fig. l can be controlled `in response to the temperature of the furnace;

Figs. 3 and 4 are a schematic wiring diagram and a simplified line diagram of another embodiment of my invention; and

Figs. 5 and 6 are simplied line diagrams lof embodiments'of my invention utilized in a threephase power supply.

Referring to Figs. 1 and 2, an electric furnace 2 is represented by insulating walls 4 defining a heating chamber '6. The furnace is heated by means of a plurality of heating resistors B, I 6 and I2. I use the term heating resistors in a somewhat generic sense as including a single heating 'element acting as a heating unit or a plurality of heating elements acting together as a heating unit.

In accordance with my invention, the heating resistors 8, I0 and I2 `are connectable in different circuit arrangements placing them in series with each other orin parallel with each other across a source of alternating-current electrical power.

The A. C. power can be taken from a pair vof terminals I4 and I6, through power supply conductors I8 and 20. In accordance with my invention, there is no regulating meansprovided 'for the power supply to the heating resistors lwhen they are connected in parallel. However, when they are connected in series, a current regulating ,A

means is inserted in series with them.

To effect the different circuit arrangements, a plurality of branch circuits are-provided 'as follows: a first branch circuit 22 which kincludes conn tacts 24 of a contactor 26 having an 'electr'omaga contacter 44 having an electromagnetic operating coil 46; and a fourth branch circuit "4B which includes contacts 5U of a contactor 152 having an electromagnetic operating coil 54 and includes a variable impedance A. C. winding 56 of a saturable core reactor 58. The contacts et and the winding 56 `are in series. `rable core reactor has a direct-current control winding 59.

Each of the contactors 26, 34, 44 and-52 ris ofa type which is normally biased to have its contacts open. Upon energization of its electro-'magnetic operating coil, the associated contacts close and complete the branch circuit in which they are connected. Y

,An end of the rst branch circuit 22, anend of the heating resistor 8, and the supply conductor I8 are directly electrically tied together to provide, in eiiect, a junction 60. The other end of this `branch circuit 22, an end ofthe heating resistor I0, 'and an end of the heating resistor I2 The satuare also electricallyitied together to provide, in effect, a junction 62. Considered differently, this branch, circuit 22 is 'connected between and across the ends of the heating resistors 8 and Ill, While the opposite ends of these resistors are directly tied together.

An end of the second branch circuit 30, the 'remaining ends of the heating Vresistors 8 and I0, and an end of the third branch circuit 40 are electrically tied together to provide, in eect, a junction 64. The other end of the branch circuit 36, the remaining end of the heating resistor I2, and an end of the fourth branch circuit 48 are electrically tied together to provide, in effect, a junction 66. Considered from a different viewpoint, th'e branch circuit 30 can be said to be connected across the ends of the heating resistors I0 and I2, while the opposite ends of these heating resistors are directly tied together.

The third branch circuit 40 has an end tied Ato the `aforesaid junction 64,- and its -oth'er end electrically tied to the supply conductor 26 and to an end of the fourth -branch circuit 46 to provide, in effect, `a junction`68.

The `fourth branch circuit 48 extends between the junction 66 and the junction 68.-

'Ihe D. YC. control winding A59 ofthe saturable core reactor S58 is in a D. C. circuit 74'. The current in the D. C.-circuit T4 is controlled in accordance with the temperature-of the heating chamber 6 by means of a temperature responsive device 16, in the form of a thermocouple, and a iD. C. current varying apparatus I8v of any suitable-type, but I prefer that shown `in the aforesaid Puchlowski patent. Briefly, in a particular system of this kind, as shown in Fig. 2, vthe thermocouple 'I6 v.controls a polarized lcontrol relay BI! having a contact 82. The operation of the relayel "and hence the contact 82 are determined through a comparison of the E. M. F. of theY thermocouple 76 with that of a known vvoltage source, as Ydescribed in `the aforesaid patent.

Assume that the contact 82 is closed to the left when :the thermocouple 'I6 .indicates v.that the temperature in the furnace is less than .the desired Vahle. Under `this condition, Van energizing circuit 84 to an operating coil i436 of an auxiliary relay 86 is closed. The upper contacts 96 of the relay 88 are bridged yand complete an energizing circuit 92 for the electromagnetic operating coils 2S, and of theccntactors. 34 and 44. Consequently, 'branch-circuits y22, `3? and 40 are closed.. With the auxiliary relay '38 energized and its contacts in upper position, the lower contacts 94 of the relay are open, so lthat a circuit 96 to the operating coil 54 of the con- .tactor `52 is not energized. The contacts 50 are open and the branch circuit 48 is vnot closed.

Under the foregoing status, the circuit arrangementacross the supply vconductors I 8 and .26 is as follows: One end of .the heating vresistor B is directly connected to the .junction :66, vand its other end is directly kconnected 5to theA 'junction 64. Similarly, one endof 'the heating resistor I0 is connected tothe junctionGIl through 'theclosed branch circuit 22, rand its other end is connected directly to the junction 264. One end of the heating resistor t2 is also connected to the junction "6d through the branch circuit 22, and its other end is-connected to the junction 64 through-the closed branch circuit 36. -I-Ience, all the heating-resistors have ends connected to the junction 6!) and the remaining ends connected to the junction 64, and are in parallel. The junction 6l) is directly connected 'to the supply-conductor I8, vand the junction 64 is connected to the supply conductor 28 through closed branch circuit 40.

As is known to one skilled in the art, the heating resistors 8, I0 and I2 in parallel will draw the highest possible current from the supply conductors |8 and 20 as compared to the current drawn by any other circuit-arrangement of the heating resistors; and the furnace is supplied with power at the highest rate possible. It is also to be observed that no current passes through the branch circuit 48 that has the A. C. winding 56 of the saturable core reactor 58 in it. This branch circuit 48 is, in effect completely short-circuited by the completed branch circuits 30 and 40, and further is itself open at the contacts 50.

When the temperature responsive device 16 indicates that the furnace is near, at or above the desired level, the Contact 82 can be made to move either1 to a neutral position or to the right with respect to Fig. 2, so that the auxiliary relay 88 is deenergized. As a result, the circuit 92 is deenergized and the circuit 96 energized. The contacts 24, 32 and 42 open, and the branch cirm cuits 22, 30 and 40 are interrupted, thereby destroying the parallel-circuit arrangement of the heating resistors 8, I0 and I2, previously described. The operating coil 54 of the contactor 52 is energized. The contacts 50 thereupon close and complete the branch circuit 43. The electrical heating resistors 8, I0 and I2 are now seriesconnected in a series-circuit arrangement with the A. C. winding 56 of the saturable core reactor 58, as follows: The junction 60, the heating resistors 8, I0 and |2 in series, the junction 66, the closed contacts 50, the A. C. winding 56 and the junction 68 to the supply conductor 20. Accordingly, the regulation of the regulating means 'i8 and saturable core reactor 58 comes into action and regulates the current supplied to the heating resistors and hence the heat to the furnace n is correspondingly regulated by the current-ccntrolling elect of the A. C. winding 56.

It will be thus observed that the A. C. winding 56 of the saturable core reactor 50 is connected to the power supply conductors I8 and 20 only when the heating resistors 8, I0 and I2 are in series. Hence, the maximum current through this winding will be less than that supplied to the furnace under any parallel-circuit arrangement which includes two or more of the heating resistors 8, I0 and I2. Not `only will the current that the A. C. winding 56 is called upon to handle be less, but the voltage drop across the winding will be much less than the full voltage across the supply conductors, because of the voltage drop across the resistors.

To illustrate the application of my invention, assume that the furnace is rated at 250 kva. and that the voltage across the supply conductors |8 and 20 is 550 volts. At a power factor of unity, 45() amperes will now in the supply conductors when the heating resistors 8, |0 and I2 are in parallel, and the resistance of each heating resistor is 32/3 ohms. When these heating resistors are connected in series. a total of l1 ohms is presented, and the maximum current that possibly can flow in the series circuit, under the assumption that the impedance of the A. C. winding 55 could be reduced to zero, is 55 amperes.

If the A. C. winding 56 were connected in series with a parallel arrangement of the resistors 8, |0 and I2, it would have had to be able to carry the full-rated current of 450 amperes and to withstand almost 550 volts. On the basis of cost, a control-reactor for such power would cost comprising a pair only of heating resistors |00 and |02. Each of these heating resistors comprises a plurality of smaller resistor sections, the former comprising two sections |00a and |001) connected in parallel; and the latter comprising two sections |02a and |0217 connected in a parallel branch in series with a third resistor section |020. Fig. 4 is a simplied diagram.

Power is derived from supply conductors |04 and |06. A first branch circuit |08 is connected to the conductor |04 and to a pair of ends of the heating resistors |00 and |02 which have their remaining or opposite ends directly tied together. The branch circuit |08 includes the contacts ||0 of a contactor |I2 having electromagnetic operating coil ||4.

A second branch circuit ||6 has one end tied to the supply conductor |06, and the other end tied to the remaining ends of the heating resistors |00 and |02. This second branch circuit I|6 includes contacts ||8 of a contactor |20 having an electromagnetic operating coil |22.

A third branch circuit |24 has an end tied to the supply conductor |06, and its other end to an end of the branch circuit |08, as shown in Fig. 3. The third branch circuit |24 includes the variable impedance A. C. winding |26 of a saturable core reactor |28, and contacts |30 of a contactor |32 having an electromagnetic operating coil |34. The saturable core reactor |28 has a D. C. control winding |36.

Fig. 3 also differs from the embodiment shown in Figs. 1 and 2 in that separate temperature responsive means are provided for controlling the circuit arrangement between the heating resistors and for controlling the impedance of the saturable core reactor |28. To this end, a temperature responsive device |40, which corresponds to the thermocouple 16 of Fig. 2, operates a regulating means |42 that regulates the current in a D. C. circuit |44 for the direct-current control Winding |36 of the saturable core reactor |23.

Means for controlling the connection-arrangement of the heating resistors |00 and |02 c-omprises a separate temperature responsive device |46 that takes a measure of the heating needed by the furnace and correspondingly controls a control device |50 in the nature rof a relay with contacts |52. The temperature-reponsive device |48 causes the control device |50 to raise its contact |52 when the temperature falls below a predetermined value, and to drop its contact when the temperature rises to a desired value somewhat below the operating range of the furnace.

With the temperature low and the contact |52 raised, an energizing circuit |54 for the electromagnetic operating coils ||4 and |22 of the contactors I I2 and |20, respectively, is energized; whereas a circuit |56 for the operating coil |34 of the contactor |32 is deenergized. Consequently, the branch circuits |08 and ||6 are closed and the branch circuit |24 is open. The heating resistors |00 and |02 are connected in parallel through circuit arrangements which should be obvious to one skilled in the art. The regulating means '|42 is entirely without eiect while the heating resistors are parallelly connected to the supply conductors |04 and |06.

When the temperature, as indicated on the temperature responsive means-|46, reaches the desired level, the control relay |50 is deenergized and its contact |52 drops, thereby deenergizing the circuit |543 for the-operating coilsl im and |-22-of the, contactors H2 and |20, respectively, and energizing the circuit |56 for the operating coil |34 of the contacter |312. As a resul-t, the contacts |3|- close, completing the branch circuit |215 and resulting in a series circuit-arrangement across the supply conductors |04, |65 as follows: the supply conductor its, the heating resistor Ilil, the heating resistor Ill-2, the contacts |36, the variable impedance A.- C winding |26, and the supply conductor |36.; Hence in this embodiment the regulating means under control of the thermocouple IM]` comes into operation only when the temperature of the furnace is within the operating range for regulating purposes as determined by the second temperature responsive device |46. The finer regulation of the furnace temperature is done through the temperature responsive device |43. It is to be observed that in this embodiment as in the earlier one, the A. C. winding |26 of the saturable core reactor operatesfor regulating purposes only when in a series-circuit arrangement with a plurality of heating resistors.

Fig. 4. is a simplinedline diagram or" Fig. 3 for indicatingr the electrical connections in a simplified form so that the relation of a diagram such as shown in Fig. 4 to a more detailed system such as shown in Fig 3 Will be more readily understood.

As will be apparent from Figs. 5 and 6, the principles of my invention can be applied to multiphase A. C. systems. Figs. 5 and 6 show three phase systems lin a simplified form after the fashion of Fig. 4. In accordance with the systems of Figs. 5 and 6, the heating resistors are connected directly to a three-phase power supply line when thev heat demand of the furnace is high, and to a single-phase of the line when the heat demand is comparatively low. Such a system would balance the load in the three phase power supply line under high demand conditions when a furnace is warming up, and impose a light single-phase demand after the furnace has warmed-up Such systems would be advantageous in connection with holding furnaces, soaking pits, baking ovens, or any other applications which are shut-down overnight or weekends and it is wished Vto bring them into operation with a minimum delay.

In Fig. 5, the furnace heating-resistors Iei, |52 and |611 are adapted to be connected directly in delta to a three-phase line comprising phaseconductors |55, |58 and |753; To this end, magnetically operated co'ntactors having contacts |12, Vid and H5 are connected as shown, with the contacts I'l2 and |74! in the power supply conductors |68V and' I'ill, respectively, and the contacts |76 in a circuit |78 tying an end of the heating-resistor 6d' to the power supply conductor When these contacts |12, |774 and |76 are closed, the heating-resistors |60, |62 and |64 are delta-connectedv to the line.

When the temperature of the furnace approaches or reaches the desired level, the contacts V52, ill and |`||i are opened and contacts |85 in a circuit |82 closed.` An obvious single phase energizing circuit is completed that extends from the conductor lill, through closed contacts wt, the A. C. winding ofA a temperature-regul'atable saturable reactor |815, and the heating resistors |54, |62 and |66 in series, to

the conductor |66'. It is, of course, tobe understood that the contacts |12, |14 and |76 are closed while contacts |8 are open, and vice versa; and the operating condition of the contacts and the saturable core reactor |84 are automatically controlled in a manner that includes the principles disclosed in connection with the prior embodiments.

Fig 6 shows, in simplied form, furnace heating-resistors connectible in Yor star to a threephase power line for high heat demand, and connectible in series across a single-phase of the line for a regulatable lower heat demand. This system has the advantage over that of Fig. 4 in that it requires one less contacter. When the contactor-contacts i8@ and |83 are closed and the contacts Iliil open, the heating resistors |92, |94 and |99 are connected in Y to the three-phase line comprising power supply conductors 2%, 2532 and 2cd, the Y connection having a neutral or star point 2%. When the contacts |86 and |83` are open and the contacts |95) closed, the heating resistors are in series across the conductors 232 and Edd through the branch circuit 2.533 which includes the contacts |98 and the A. C. winding of the temperatureregulatable saturable `core reactor 2HE. .T t is to be observed that this branch circuit is connectible and disconnectible, by the contacts |93, to ends of heating resistors |92 and ist. The series circuit can be traced as follows: the conductor 2ii2, the heating resistor |24, the branch circuit 2%, the heating resistor |92, the neutral point 268, the heating resistor |96 and the conductor While I have described my invention in forms which are at present preferred, it is obvious that the principles of my invention have wide application and can be embodied in different forms with different numbers ofA resistors.

I claim as my invention:

1. An electric heating means comprising threephase pow-er supplyy conductors; a plurality of heating resistors, there being a resistor for each phase; controllable circuit-controlling means connecting said heating resistors directly to said power supply conductors with'a heating resistor for each phase; additional controllable circuitcontrolling means, including a branch circuit having the alternating-current winding of a saturable core reactor, cooperating with the rst said circuit-controlling means for changing the aforesaid connections of said heating resistors and connecting all of said heating resistors solely in series with said branch and across a pair of said power supply conductors, said saturable core reactor having a D. C. control winding; and temperature-responsive means selectively operable on said circuit-controlling means, and controlling the current in said D. C. control winding.

2. An invention including that of claim 1 but characterized by said circuit-controlling means comprisingr a pair of contacts in each of a pair of said power supply conductors operable to place the rst said heating resistors in delta across ther power supply conductors.

3. An invention including that of claim 1 but characterized further by said circuit controlling means being operable to place the first said heating resistors in v across the power supply conductors, and said branch circuit being connected to an end of a rst heating resistor which is connectible to one of said power supply conductors, and to an end of a second heating resistor which is connectible to the star point of the Y.

4. An electric heating means of a type described comprising, in combination, `a saturable core reactor having a variable impedance winding and a D. lC. control Winding, a plurality of A. C. supply conductors, a plurality of heating resistors, the number of heatingresistors being at least as large as the number of said supply conductors, a direct connection between an end of a first heating resistor and an end of a second heating resistor, a branch circuit connecting a rst of said supply conductors to the lother end of said iirst heating resistor, said branch circuit including openable and closeable contacts, a second-branch circuit from said first supply conductor to the other end of said second heating resistor, said second branch circuit including, in series, openable and closeable contacts and the variable impedance Winding of said saturable core-reactor, and temperature-responsive means operable for operating said contacts so that the first of said contacts is open while the other is closed, and vice versa.

5. The electric heating means according to claim 4, in which said temperature responsive means is operable through said contacts to connect a number of said plurality of heating resistors in a plurality of different parallel circuits across said alternating-current supply conductors, or to connect all of said heating resistors solely in a series circuit across a pair of said alternating-current supply conductors.

6. The electric heating means according to claim 4, in which there are provided circuits including said contacts selectively operable to different positions for connecting all of said heating resistors solely in a series circuit which includes said variable impedance Winding or in a parallel circuit arrangement excluding said variable impedance winding, and said temperature responsive means automatically controlling the selective operation of said contactors and contacts.

'7. The electric heating means in accordance With claim 4 for use in an electric furnace, there being provided a Variable current direct-current circuit which includes said direct-current control winding, and said temperature responsive means includes a device responsive to the temperature of said furnace for controlling the current in said variable current direct-current circuit, whereby to regulate the impedance of said variable impedance winding.

8. The electric heating means in accordance with claim 4, for use in an electric furnace system, said plurality of heating resistors being used to heat said furnace there being provided a plurality of tying conductors including said direct connection directly connecting an end of each of said heating resistors to an end of a different one of said heating resistors, a third branch circuit including separable contacts across the opposite ends of said heating resistors, and another of said supply conductors being connected to the other end of said first heating resistor.

9. The heating means including that of claim 8 but further characterized by said temperature responsive means which is operable to place part of the separable contacts or said third branch circuit in open position when the openable and closable contacts are in open position in said branch circuit connecting the iirst of said supply conductors to said other end of said first heating resistor, and which is operable to place the latter said contacts and said 1fart of said separable contacts in closed position together.

10. An invention including that of claim 9 but further characterized by a Variable control circuit for said control Winding, and said temperature responsive means including means regulating said variable control circuit, whereby to vary the impedance of said variable-impedance Winding.

11. The electric heating means in accordance With claim 4, in which there are provided a Lplurality of circuit-connections including contactors and said openable and closable contacts associated with said heating resistors and selectively controllable to connect a number of said resistors in a plurality of diierent parallel circuits across a plurality of said alternating-current supply conductors, or to connect all of said heating resistors solely in a series circuit across a pair of said supply conductors, depending on the positions of said contactors, means for selectively positioning said contactors so as to place said variable impedance winding in said series circuit, and to exclude it from said parallel circuits, and said temperature-responsive means controlling said variable impedance winding.

12. Apparatus according to claim 4 characterized by the fact that the contacts of the rst and second branch circuits are associated With said heating resistors and selectively operable t0 connect a number of the heating resistors in a plurality of different parallel circuits across a Plurality of power supply conductors or to connect all of said heating resistors solely in series circuit across a pair of said power-supply conductors, depending on the positions of said contacts and by temperature responsive means Which substantially simultaneously operates said contacts.

13. An electric furnace system comprising a furnace having a plurality of heating resistors for heating said furnace according to claim 4 characterized by the fact that the temperature responsive means automatically controls the selective operation of the contacts.

14. An invention including that of claim 13 but further characterized by a variable current direct-current circuit including the direct-current control winding of the reactor, and by temperature-responsive means which includes a device responsive to the temperature of said furnace for controlling the current in said variable current direct-current circuit, whereby to regulate the impedance of said variable impedance Winding.

15. An electric furnace system comprising a furnace having a plurality of heating resistors for heating said furnace; according to claim 4 characterized by a number of tying conductors directly connecting an end of each of said heating resistors to an end of a different one of said heating resistors; and by the fact that the rst mentioned branch circuit connects the first of the supply conductors to a rst of said tying conductors that is at an end of a rst heating resistor; and by the further fact that the second of the supply conductors is connected to the other end of the last said heating resistor; and further characterized by a third branch circuit including separable contacts across the opposite ends of said heating resistors.

16. An invention including that of claim 15 but further characterized by means controlled by said temperature responsive means for placing the last said contacts in open position and the other of said contacts in closed position, and vice versa.

l 12 :17. An .invention including that', -of Vclaim 16 REFERENCES CITED but further characterized by a Variable ucontrol The following references are of :record in ,the

circuit for said control Winding, `andsaid temme of this partent:

Aperaadure responsive meansincluding means regulating said variable controlcircuit, vwhereby to 5 UNITED 'STATES PATENTS vary the impedance of said Variable-impedance Number Name Date winding. 1,612,004 Weed Dec..28,1926 Y18. 4An Vlelectric `heating -means as defined in 1,625,380 Talley Apr. 19, 1927 claim 4 `but: .further characterized by said tem- 1,638,857 Keene Aug. 16, 1927 tpnerature responsive meansrcontrols the operation 10 1,798,678 Keller Mar. 31, 1931 of the saturable corereactor. 1,871,697 James Aug. 16, 1932 2,276,822 Bowman et a1.l Mar. 17, 1942 JAMES L- REED- 2,367,619 schneider Jan. 16, 1945 2,422,958 Embry June 24, 1947 2,445,454 PuchlOWsk July 20, 1948 

